Dr. Bob on Electrical/Electronic Connector Contact Spring Materials
A functional description of the performance requirements for an electrical/electronic connector is that it must transfer an electronic signal or electrical power between two sub-units of an electrical/electronic system without unacceptable power loss or degradation in signal integrity. Also, it must maintain this capability consistently over multiple mating cycles (separability). In practice, meeting these requirements is accomplished by creating a metallic contact interface between the two halves of the connector, usually a plug and receptacle.
The resistance of a contact interface depends on the area of the interface and the conductivity of the materials in contact. A simplified expression of this relationship, for a circular unplated contact area, is:
RContact = r ( H / FNormal)1/2
where RContact is the resistance of the contact interface, r is the resistivity of the contact material, H is the hardness of the contact material, and FNormal is the contact normal force—that is, the force perpendicular to the contact interface. The general form of this equation applies to contact interfaces of geometries other than circular.
Note that the contact material is referenced directly in both r and H. The contact material is also implicit in the FNormal term, because it is the deflection of the contact springs during the mating of the connector which generates the contact normal force.
For electrical/electronic connectors, minimizing resistance—both bulk and contact—is important. Thus a low-resistivity, high-conductivity metal is desirable. The resistivity of metals varies significantly depending on the material and its processing, both mechanical and chemical. For example, copper, aluminum, and iron have resistivities (in microohm-centimeters) of 1.67, 2.65, and 9.71, respectively. Mechanical processing, drawing, rolling, and forming can slightly increase the resistivity of a metal. Alloying is more significant when chemistry is altered. For example, copper alloy C26000, a cartridge brass commonly used as a connector spring material, has a resistivity of 6.2 microohm-centimeters.
Now consider the H / FNormal term in the equation. As mentioned, H is the hardness of the material. The “appropriate” hardness for a connector is a compromise between the benefits of low hardness, which allows for a larger contact area (lower resistance) for a given contact force, and the benefits of high hardness, which allows for better wear performance (more mating cycle capability).
The FNormal term is a bit more complicated. There are two equations relating contact normal force to material and geometric parameters.
F = f1 [E, D, geom.]
F = f2 [s, geom.]
The functions f1 and f2 depend on the geometry of the contact beams. D is the deflection of the receptacle beam on mating. Our focus is on the terms E, the elastic modulus of the material, and s, the stress in the deflecting beam.
E is a material property that varies slightly with alloying. Copper and copper alloys have values of E in the range of 16 to 22 (all E values are in million psi). Aluminum and aluminum alloys range from 8 to 11. So while elastic moduli do vary, they lie in a relatively narrow range. Steels have an elastic modulus of around 30.
The yield strength shows a wider range of variability, particularly due to alloying. The reasons for this variability will be discussed metallurgically in the next article in this series. For now, you need to know that copper alloy yield strengths range from 30 ksi to nearly 200 ksi, with the majority of copper alloys falling in the 70 to 120 ksi range.
There is another contact spring performance requirement that has not yet been mentioned. As noted, a connector is used to make a separable connection between two sub-units of an electronic product. Each half of the connector is attached, often permanently, to a printed wiring board or cable. Therefore, the contact spring material must withstand forming or soldering processes needed to make these permanent connections. Mechanically, this requires that the alloy be formable to make crimped and IDC connections, as well as the forming operations necessary to create the varying geometries of receptacle contacts.
Therefore, the ideal contact spring material would have a low resistivity, and a balance between the strength and resilience necessary to generate acceptable contact forces and the formability necessary to create permanent connections as needed.
Copper alloys are the most commonly used contact spring materials because they provide the best balance of these electrical and mechanical properties at an acceptable cost. The high conductivity of copper and copper alloys places it well above other materials, with the exception of silver, in this property. The mechanical properties of copper alloys are adequate for most connector applications. Copper alloys also exhibit a good formability to strength ratio, compared to many other metals.
I’ll discuss the basics of copper alloy metallurgy, as it is applied to connector contact spring applications, in my next article, and wrap up the series with some specific connector materials selections guidelines that should help you make materials decision with greater knowledge and confidence. Stay tuned.
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- Dr. Bob on Gold Flash Contact Finishes (and Max Peel) - September 22, 2014